I searched, but haven't found an answer... Typically on my routes, I select everyday. Although the monthly "c" check puts my (as of now my route and aircraft) out of service for a month. I've attempted to put another aircraft on that route for that month, but can't find that option. From what I understand, I would have to close that route for that month, replace it with another aircraft/route, then re-open it after the "c" check. Let me know, maybe there is a way, or maybe it's very trivial.
select the route
click the + sign to duplicate it
assign that to your other plane
wash. rinse. repeat.
If you have a plane of the same fleet group with empty schedule just to go schedule and click on 'move schedule' icon next to the original aircraft.